Present environmental concerns, notably with view to reducing carbon dioxide emissions, induce an urgent need for reducing fuel consumption of light duty automotive vehicles or trucks, as well as building site machines or agricultural machines. In particular, there exists an increasing demand for organs such as the engine, the transmissions, the reduction gears, the compressors and hydraulic systems which contribute to reducing energy consumption. Accordingly, the lubricants used in these organs should allow reduction of frictional and splashing losses to a level as low as possible. It is known to one skilled in the art that lowering the viscosity of the lubricant oil is a means for improving fuel savings achieved at the transmissions.
Power train (PT) bench tests including an engine/gear boxes assembly have thereby shown that fuel savings which are made are directly proportional to the viscosity of the transmission lubricant at the operating temperature, which is generally located between 20 and 40° C. for a use of the vehicles on short trips. The best performances are obtained with oils of kinematic viscosity measured according to the ASTM D445 standard, to be located at about 20 mm2/s at the operating temperature. Moreover, manufacturers' specifications systematically impose for transmission oils for private vehicles, a viscosity at 100° C. (or KV 100) as measured according to the ASTM D445 standard, comprised between 5 and 15 mm2/s, most often comprised between 6 and 9 mm2/s, preferentially targeted around 6.5 mm2/s. This limitation is related to mechanical design considerations for gear boxes, bearings, gears. Indeed, below a limiting viscosity of about 5 mm2/s, the dimensioning of the parts should be modified in order to reduce the load per unit surface, since the lubricant does not sufficiently participate in supporting said load.
The viscosity behavior of oils strongly depends on the bases used, in their formulation, in an amount of at least 50% by mass in general. Thus, the formulation of transmission oils having a strong effect on fuel savings, or further strong fuel-saving properties or so-called “fuel eco” properties, will preferentially resort to lubricant bases having a very high viscosity index or VI. The viscosity index or VI of a base measured according to the ASTM D2270 standard quantifies its capacity of limiting its temperature-dependent changes in viscosities, from the measurement of its kinematic viscosity at 40° C. (KV40) and 100° C. (KV100) measured according to the ASTM D445 standard.
The VI of known conventional mineral bases is at most of the order or 200. With certain synthetic oils, it is possible to attain very high VIs, of the order of 400, but this high VI is accompanied either by strong viscosity or by constraints on solubility of the additives, with which it is not possible to impart to the lubricant, properties for protecting the gears, for controlling friction, . . . , expected by the manufacturer. It is therefore difficult to formulate transmission oil with eco fuel properties in majority from these bases. Their cost and their availability are also a problem for large scale industrialization of lubricants incorporating them in majority.
Certain fatty acid esters of natural origin intrinsically have a very high VI of the order of 250, or even 300 and beyond, combined with low viscosity. However, one skilled in the art is not encouraged to use these esters for automotive lubricants, in particular for engines and transmissions, since the esters of this type, liquid at room temperature, have at least one double bond on their acid chain, which gives them very low resistance to oxidation, whence a risk of degradation during operation. These esters, used as bases, do not in particular satisfy high temperature oxidation tests, either catalyzed or not, which are part of the specification of automotive manufacturers for these applications.
Surprisingly, the applicant noticed that it was possible to formulate transmission oils with very high VI, of more than 250 or further 280, preferentially more than 300, or even of the order of 320 and beyond, from bases of natural origin of the fatty acid methyl ester types, and having an operating lifetime comparable with that of existing commercial products. Oils for gear boxes in particular should be designed for “filled for life” conditions, i.e. they are never emptied throughout the lifetime of the vehicle. Without intending to be bound by any theory, it seems that these esters, which form, at the surfaces of the frictional parts, films with which hydrodynamic flow conditions may be maintained under a high load, limit heating of the oils during operation. Thus, in spite of the poor results on the standard oxidation tests, the operating results are quite satisfactory.